1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a continuous-media type of perpendicular magnetic recording disk with a granular cobalt-alloy recording layer having controlled grain size.
2. Description of the Related Art
In a “continuous-media” perpendicular magnetic recording disk, the recording layer is a continuous layer of granular cobalt-alloy magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the write head writes on the magnetic material. Continuous-media disks, to which the present invention is directed, are to be distinguished from “bit-patterned-media” (BPM) disks, which have been proposed to increase data density. In BPM disks, the magnetizable material on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to continuous-media disks wherein a single “bit” may have multiple magnetic domains separated by domain walls.
FIG. 1 is a schematic of a cross-section of a prior art perpendicular magnetic recording continuous-media disk. The disk includes a disk substrate and an optional “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. The material for the recording layer (RL) is a granular ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Si, Ta, Ti, Nb, B, C, and W. These oxides (Ox) tend to precipitate to the grain boundaries as shown in FIG. 1, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. An optional capping layer (CP), such as a granular Co alloy without added oxides or with smaller amounts of oxides than the RL, is typically deposited on the RL to mediate the intergranular coupling of the grains of the RL, and a protective overcoat (OC) such as a layer of amorphous diamond-like carbon is deposited on the CP.
The Co alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hexagonal-close-pack (hcp) crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, intermediate layers of ruthenium (Ru1 and Ru2) are located below the RL. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that induce the growth of the RL. An optional seed layer (SL) may be formed on the SUL prior to deposition of Ru1.
The enhancement of segregation of the magnetic grains in the RL by the additive oxides as segregants is important for achieving high areal density and recording performance. The intergranular Ox segregant material not only decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on the Ru2 layer that exhibits columnar growth of the Ru or Ru-alloy grains.
FIG. 2 is a transmission electron microscopy (TEM) image of a portion of the surface of a prior art CoPtCr—SiO2 RL from a disk similar to that shown in FIG. 1. FIG. 2 shows well-segregated CoPtCr magnetic grains separated by intergranular SiO2 (white areas). However, as is apparent from FIG. 2, there is a relatively wide variation in the size of the magnetic grains and thus the grain-to-grain distance. A large grain size distribution is undesirable because it results in a variation in magnetic recording properties across the disk and because some of the smaller grains can be thermally unstable, resulting in loss of data. FIG. 2 also illustrates the randomness of grain locations. Because the nucleation sites during the sputtering deposition are randomly distributed by nature, there is no control of the grain locations. The amount of Ox segregants inside the RL needs to be sufficient to provide adequate grain-to-grain separation, but not too high to destroy the thermal stability of the RL. The typical content of the Ox segregants is about 20% in volume, and the grain boundary thickness is typically between about 1.0 and 1.5 nm.
To achieve high areal density of 1 to 5 Terabits/in2 and beyond, it is desirable to have high uniformity (or tighter distribution) of the grains within the RL, mainly for the following three structural parameters: grain diameter (i.e., the diameter of a circle that would have the same area as the grain), grain-to-grain distance (i.e., the distance between the centers of adjacent grains or “pitch”) and grain boundary thickness. Narrower distribution of these three structural parameters will lead to narrower distributions of magnetic exchange interaction and magnetic anisotropy strength, both of which are desirable.
Thus the prior art RL shown in FIG. 2 is far from ideal. First, the grains have an irregular polygonal shape with a large size distribution. The average grain diameter is about 8-11 nm with a relatively large size distribution of about 18-22%. The distribution information is obtained by measuring neighboring grain-to-grain distances in high resolution scanning electron microscopy (SEM) or TEM images and then fitting with a lognormal function. Distribution value as referred to in this application shall mean the width of the lognormal function. Second, the location of the grain centers is highly random, which means there is no short range or local ordering, i.e., no pattern within approximately 3-5 grain distances. Third, the thickness of the grain boundaries (the Ox segregants seen as white areas in FIG. 2) has an even wider distribution. Typical grain boundary thickness is 0.9-1.2 nm with a distribution of about 50 to 70%. Because the intergranular exchange is an exponential decay function of boundary thickness, the large distribution of boundary thickness leads to large fluctuation of exchange between grains and thus a significant signal-to-ratio (SNR) loss. According to Wang et al, “Understanding Noise Mechanism in Small Grain Size Perpendicular Thin Film Media”, IEEE Trans Mag 46, 2391 (2010), a large distribution of boundary thickness can cause 3 to 10 dB SNR loss, and small-grain media suffer even more SNR loss than large-grain media.
What is needed is a continuous-media perpendicular magnetic recording disk that has a granular cobalt alloy RL with additive oxides with well-segregated magnetic grains with a narrow distribution of grain diameter and grain boundary thickness.